1. Field of the Invention
The present invention relates to a solid-state imaging apparatus carrying out the operation of skipping a part of effective pixels to read out the part, and an imaging system including the solid-state imaging apparatus.
2. Description of the Related Art
In recent years, video cameras and still cameras, each being mounted with a complementary metal oxide semiconductor (CMOS) sensor superior in reading out signals at high-speed, have been increasing. Moreover, also cameras have been developing to have multiple functions, and sensors are required to perform a plurality of read out methods. The sensors are required, for example, to switch their sensing modes among a still image mode wherein the ratio between the lateral direction and longitudinal direction of a screen is 4 to 3, a high definition (HD) moving image mode wherein the ratio between the lateral direction and longitudinal direction of a screen is 16 to 9, and a moving image mode of the ratio between the lateral direction and longitudinal direction of a screen is 4 to 3, for example, Video Graphics Array (VGA), and to realize electronic zooming. As a means for switching the resolution of an image to be obtained, that is, the number of pixels from which signals are read out, there is a thinning out operation. In the thinning out operation, it is possible to obtain an image having a resolution lower than the maximum resolution of an imaging device by selecting the pixels from which signals are read out every one or a plurality of lines (rows or columns).
Generally, there is a pixel region called an optical black pixel region (hereinafter referred to as an OB region) around an effective pixel region to be used for image formation. The OB region is used for signal correction, such as offset elimination, and the OB region includes the pixels each having a photodiode which is a photoelectric conversion element and is shielded from light. Generally, signal correction using a plurality of pixels in the OB region (hereinafter referred to as OB clamping) is carried out. The pixels in the OB region will hereinafter be referred to as OB pixels. In the OB clamping, signals are read out from the plurality of pixels in the OB region and are averaged. However, if the OB pixels are also thinned out when signals are read out from the effective pixels by the aforesaid thinning out operation, then there is the possibility that noises are not sufficiently reduced.
Against this problem, there is a technique proposed in Japanese Patent Application Laid-Open No. H09-163236. According to Japanese Patent Laid-Open No. H09-163236, OB pixels are read out in a normal read out mode, in which pixels are read out without being thinned out, even when light receiving pixels in the effective pixel region are read out in a thinning out reading out mode, in which pixels are read out by a thinning out operation. In addition, it is described in the publication to drive the light receiving pixels and the OB pixels at different frequencies. Japanese Patent Application Laid-Open No. H09-163236 describes that it is thereby possible to realize an OB clamping operation with high accuracy without increasing the number of the OB pixels even in the thinning out reading out mode.
However, the method of the publication has the following problem.
Generally, in a CMOS sensor used for moving imaging, the charge accumulation time periods of pixels are controlled by rolling shutter operations. In the rolling shutter operations, a row to be reset and a row from which signals are read out are sequentially shifted, and thereby a time period from resetting to reading out is set as an accumulation time period.
FIG. 14 illustrates the timing at the time of applying the rolling shutter operations to an imaging device including OB pixels on four rows in the upper part thereof and effective pixels on and after a fifth row. The abscissa axis of the graph illustrated in FIG. 14 indicates time, and the ordinate axis thereof indicates the positions of pixel rows. FIG. 14 illustrates OB pixel row reading out time periods 39, effective pixel row reading out time periods 40, effective pixel row skipping time periods 41, during which no signals are read out, that is, the pixels to be thinned out are scanned, OB pixel row resetting time periods 42, and effective pixel row resetting time periods 43. A time period from a time t1 at which the resetting of the effective pixel rows from which signals are read out ends to a time t2 at which reading out is started is the charge accumulation time period of pixels. Because both of the OB region and the effective pixel region are driven by the same drive pulses here, each of the effective pixel row reading out time periods 40 and each of the effective pixel row skipping time periods 41 are the same lengths. Consequently, the charge accumulation time period on each row does not shift from one another.
There is a requirement of shortening the total read out time needed for reading out the signals of all the pixels in order to increase the number of imaged screens per time. It is accordingly conceivable to shorten the effective pixel row skipping time periods 41 in order to shorten the total read out time of all of the pixels, but another problem arises in this case. The charge accumulation time periods, which are set by the rolling shutter operations are different from one another on the respective rows. This problem is caused because the control of reading out signals and the control of carrying out resetting are carried out with the same patterns of drive pulses. Thereby causing the reset time periods of the respective rows to be different from one another to make the charge accumulation time periods uneven.